Senior forecaster at Weatherzone, Brett Dutschke, told 891 Breakfast the record breaking 13 days over 40 degrees in summer would be broken when a low pressure system sweeps the southern parts of the state tomorrow.

"Widespread 40 to 80mm [falls] with the potential for near 100mm in some parts."

Whilst many areas of the state will welcome the falls, the heavy rain could decimate crops for local grape growers.

"They are in harvest mode and this is really going to spoil some of the grapes," Brett said.

"They'll split and be prone to disease."

He said areas north of Adelaide, particularly in the Barossa Valley and Mount Lofty Ranges, could expect the highest falls.

"This is going to lead to some flooding."

Potential flooding could be reduced in areas expecting heavy rainfall by clearing leaves and debris from gutters and drains before the system approached.

Light rain was expected tomorrow before the heavier falls approached on Friday.

"Within Saturday the system will spread north and east, so northern parts of the state will get a fair bit of rain too."

Record heat for summer

Adelaide entered its 13th day over 40 degrees with an overnight low of 31.3 at 6:00am, with a forecasted top of 43 degrees expected.

The 117-year-old record was broken yesterday when Adelaide reached over 40 degrees for the 12th time in a summer season, peaking at 41.9 at 3:30pm.